1. Field of the Invention
The invention generally relates to capacitor start single phase induction motors and, in particular, to a 2/4 pole motor employing a partial 2-pole winding starting and to 4/8 and 4/6/8 pole motors employing a partial 4-pole winding starting.
2. Description of the Prior Art
Capacitor start single phase induction motors having two or more pole configurations are useful in multi-speed applications. For example, some washing machines employ 4/6/8 pole capacitor start single phase induction motors because such motors are able to adjust speed for various cycles within a certain speed range without the need for a complicated and expensive variable frequency controller. There is a continuing need to design such motors to provide good performance for both running and starting at a low cost of materials and manufacture. The running performance includes evaluating such factors as torque, speed, current, temperature rise, efficiency, size and other factors. The starting performance includes evaluation of the speed-torque curves of the motor. The minimum torque or torque dip at a speed between zero to the speed at breakdown torque can be a limiting factor for starting. Sometimes, a compromise needs to be made among the running performance, starting performance, material cost, tooling or other factors.
Some 4/6/8 pole motors in production employ basically independent 4-pole, 6-pole and 8-pole main windings and a 4-pole auxiliary winding. Only a small portion of a 6-pole winding is shared with a 4-pole winding. To reduce the size of the motor and save active material, a new approach to sharing the 4-pole and 8-pole windings has been developed and is described in co-invented, co-assigned PCT patent application Ser. No. PCT/US99/11235, filed May 20, 1999, entitled SINGLE PHASE THREE SPEED MOTOR WITH SHARED WINDINGS, the entire disclosure of which is incorporated herein by reference. One advantage of the shared winding approach is that it improves running performance without an increase in material cost. However, to reconnect the main winding from a 4-pole configuration to an 8-pole configuration, or visa versa, one more contact, such as a single pole single throw switch, is needed either as part of the starting switch or as part of the external circuit of the motor. If the additional contact is implemented as part of the starting switch, the starting switch will require three switches and four contacts. This implementation makes the starting switch more complicated and expensive to retool and produce. On the other hand, if the additional contact is implemented as part of the external circuit of the motor, the additional contact would be in the form of a timer or a relay. However, since the 4-pole to 8-pole reconnection is implemented by the external circuit, the motor cannot use 4-pole starting for 8-pole running without affecting the starting performance. Using 6-pole starting can avoid this problem (see PCT/US99/11235 noted above). However, when the motor employs 6-pole starting and 4-pole running, a torque dip occurs during the transfer from 6-pole to 4-pole. To cancel the dip, a time delay relay is required to maintain the 6-pole auxiliary winding in the circuit 40 ms longer during transfer from starting to running. The relay is an additional component providing additional cost.
There is a need for a 4/8 pole and 4/6/8 pole motors which employ 4-pole starting so that many of the above problems are avoided. There is also a need for a 2/4 pole motor which employs 2-pole starting so that many of the above problems are avoided.